All the trees in the Singapore Botanic Gardens' Palm Valley have been found to be safe after a detailed check, with more inspections to follow, said the National Parks Board (NParks) yesterday, a day after a falling tembusu tree killed a woman and injured several others.
NParks said it will conduct more checks on trees in the vicinity of the Palm Valley, such as the Rainforest, and on all the heritage trees in the Gardens.
"We want to assure the public that we share concerns about the safety of our trees in the Singapore Botanic Gardens, especially in view of the recent spate of intense weather conditions," said its commissioner of parks and recreation Leong Chee Chiew in a statement.
While Palm Valley, which was the site of the incident, remains closed to public access owing to clearance operations and investigations, the agency declared the rest of the Gardens to be safe for the public to visit.
The 40m-tall tembusu heritage tree killed Indian national Radhika Angara, 38, last Saturday after it collapsed, bringing down nearby palm trees along with it. Her French husband, Mr Jerome Rouch-Sirech, 39, and their one-year-old twin children - a boy and a girl - were also injured. A Singaporean woman, Ms Tay Pei Lei, 26, was also hurt.
Following online chatter questioning whether tree inspections by NParks were rigorous, the agency assured the public yesterday that it has a comprehensive tree management programme that is also designed to cope with unpredictable and severe weather patterns.
The Tembusu tree that fell was inspected twice a year, which is of a higher frequency than for other trees in the Gardens, where checks are done on the root collar, anchoring roots, crown, trunk and signs of soil movement, said Dr Leong.
He added that the tree was also protected by a lightning conductor and fenced off to prevent compaction of its root zone by visitors, and that leaf litter was routinely applied to the root zone to encourage healthy root growth. The tree was found to be healthy during its last inspection in September last year.
The frequency of inspections for trees along expressways and major roads has also been stepped up from once in 12 to 18 months, depending on tree species, size and location, to once every six to 12 months since 2012. As of November last year, NParks introduced yearly detailed second-level inspections of trees of more than 4m in girth.
Dr Leong added that NParks has taken measures to improve the general health of its trees as well, including through regular application of fertilisers and pruning.
Extra care is also taken to prune and conduct crown reduction prior to periods of more severe weather conditions - done on top of its normal tree pruning programme.
Going forward, NParks is developing modelling techniques to better understand the structural behaviour of trees under varying environmental conditions like rain, wind and soil, said Dr Leong.
The agency also cautioned against speculating on the causes of the tree fall and to allow due process to take its course.
Singaporean plant expert Jean Yong, an eco-physiologist at the Australian Research Centre for Mine Site Restoration, said the root system of a tree that size could spread over 50m to 100m.
The centre works on research projects underpinning successful mining restoration outcomes. Trees are a key component of ensuring such sites can be rehabilitated.
"Issues relating to urban trees are challenging and require good scientific allometric assessment to better predict tree stability when the soil is very wet, and when encountering a seasonal microburst of strong gale-force winds," Dr Yong said.
Besides tree health, which routine inspections now focus on, constraints to root space volume should also be recognised and documented, he added.
• Additional reporting by Audrey Tan